Manufacturing of electronic wafer chips involves a step wherein semiconductor work-pieces are cleaned with a liquid solution during or after Chemical Mechanical Planarization (CMP). A “semiconductor work-piece”is a microelectronic device that has not completed the fabrication process, typically a silicon wafer with active regions formed in or on the surface of the silicon wafer. Connections to the active regions are made using multiple layers of metal, typically copper and tungsten, which has been deposited on the silicon substrate. When copper is used as the interconnect material, a damascene process is used whereby the copper is deposited into lines etched into the inter-layer dielectric and then the excess copper is removed and the surface planarized using a CMP process, followed by a cleaning step. The goal of the cleaning process (“Post-CMP cleaning”) is to remove residues left by the CMP step from the semiconductor work-piece surface without significantly etching the metal, leaving deposits on the surface, or imparting significant contamination to the semiconductor work-piece. Furthermore, it is desirable to protect the metal surfaces from corrosion by various mechanisms such as chemical etching, galvanic corrosion or photo-induced corrosion. Corrosion of the metal surfaces results in metal recess and thinning of the metal lines. Since neutral to alkaline slurries are often utilized in copper and barrier CMP, it is desirable to have a cleaning solution that is effective in the alkaline pH regime where the abrasive particles are highly charged and can be removed efficiently. Alkaline chemistries are often utilized in a brush scrubber or megasonic cleaning unit for Post-CMP cleaning.
A cleaning solution may contain various chemicals that perform different functions during the cleaning process. A cleaning solution must contain a “cleaning agent.” A “cleaning agent” is the component of solution that removes residual CMP slurry particles, typically particles of metal, from the surface of the semiconductor work-piece. A cleaning solution may also contain “chelating agents,” “corrosion-inhibiting compounds,” and/or “surface-active agents.” A “chelating agent” helps prevent re-deposition of removed metal onto the semiconductor work-piece by complexing the metal in the cleaning solution. A “corrosion-inhibiting compound” is the component of the cleaning solution that protects the metal surface from attack by mechanisms such as the aggressive nature of the cleaning solution, oxidation, post cleaning corrosion, galvanic attack, or photo-induced attack. A “surface-active agent” is a component of the cleaning solution that modifies the wetting characteristics and prevents watermark formation.
U.S. Pat. Nos. 6,200,947, 6,194,366, and 6,492,308 disclose relevant to the chemistries of cleaning solutions. However, these references suffer from one or more of the disadvantages discussed below.
The ability of the cleaning chemistry to remove residual metals and retain them in the cleaning solution is an important characteristic of a Post-CMP cleaning solution. Chemicals that can complex the residual metal in the cleaning solution are effective cleaning agents because the residual metals are not re-deposited on the semiconductor work-piece after they are removed. Cleaning solutions using chemistry that cannot complex the residual metals typically perform poorly at the desired cleaning task. Thus, it is desirable to have a cleaning solution that includes a chelating agent.
Some commercially available alkaline chemistries have poor performance at the desired function of removing residual metal, particularly copper, from dielectric lines because the do not contain a chelating agent. Such chemistries typically comprise an aliphatic alcohol compound containing a mercapto group, such as 2-mercaptoethanol or thioglycerol, and a solution of an alkaline compound, such as a hydroxide.
It is important to protect semiconductor work-pieces from corrosion of the metal surfaces by providing a corrosion-inhibiting compound in the cleaning solution. The metal surfaces of the semiconductor work-piece, typically copper, form the conducting paths of the semiconductor wafer. Due to the very small size of features on semiconductor wafers, the metal lines are as thin as possible while still carrying the desired electric current. Any corrosion on the surface or recess of the metal causes thinning of the lines (dissolution) and results in poor performance or failure of the semiconductor device. The cleaning solution's corrosion preventing abilities are quantified by measuring the static etch rate or the surface roughness (quantified by RMS, root mean square, value) of a metal surface that has been cleaned with the subject solution. A high static etch rate indicates dissolution of the metal surface is occurring. A high RMS value indicates a rough surface caused by attack of the metal at the grain boundaries. An effective corrosion-inhibiting compound reduces the corrosion of the metal as indicated by lower static etch rate and RMS values measured after the cleaning step.
Corrosion-inhibiting compounds function by either reducing the surface of the metal, providing a protection film on the surface of the metal, or scavenging oxygen. Some cleaning solutions available in the art do not provide effective corrosion-inhibiting agents and thus suffer from a high static etch rate and/or high RMS value.
Some commercially available alkaline cleaning chemistries are affected by exposure to air and/or have a high static etch rate of the metal. Such chemistries typically contain a quaternary ammonium hydroxide, such as TMAH, a corrosion-inhibitor of the oxygen scavenging type, such as gallic acid or ascorbic acid, and an organic amine, such as monoethanolamine. Because these chemistries rely on an oxygen scavenger to prevent corrosion, exposure to air is detrimental to the performance of the chemical. Also, the lack of a protective surface film and the aggressiveness of the chemistries towards the metal result in a high static etch rate, which then causes recessed lines.
Another common problem with cleaning semiconductor surfaces is the deposition of contaminants on the surface of the semiconductor device. Any cleaning solutions that deposit even a few molecules of undesirable composition will adversely affect the performance of the semiconductor device. Cleaning solutions that require a rinsing step can also result in depositing contaminants on the surface. Thus, it is desirable to use a cleaning chemistry that will not leave any residue on the semiconductor surface.
It is also desirable to clean and protect the semiconductor surface in a single step. Some chemistries for planarizing a wafer surface include a cleaning step followed by an additional step of rinsing with water or an inhibitor solution. Rinsing, particularly with water, can result in leaving deposits on the surface of the semiconductor work-piece, thus contaminating the wafer. Adding a second step is also a drawback due to the fact that it lengthens the manufacturing process, complicates the process by having to handle more chemicals and more steps, and provides one more possible source of contamination or other quality control problems. Clearly, a process that cleans and protects the surface of the semiconductor work-piece is desirable.
It may also be desirable to have a surface wetting agent in the cleaning solution. Surface wetting agents prevent contamination of the semiconductor work-piece by helping to stop spotting of the surface caused by droplets clinging to the surface. Spotting (also called watermarks) on the surface can saturate metrology tools that measure light point defects, thus masking defects in the semiconductor work-piece.
For the foregoing reasons, it is desirable to provide an alkaline chemistry that protects the metal from corrosion, prevents oxidation of the metal surface, removes particles efficiently, removes metal from the dielectric surface, is close to the pH of the prior CMP step, and does not contaminate the semi conductor surface. The chemistry of the current invention makes use of multiple additives to provide a solution that fulfills all of the above needs.